1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus for use in, for example, a filter having an unbalanced-to-balanced conversion function, and also relates to a communication unit using the SAW apparatus described above.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and weight of communication units, such as cellular telephones. As the frequency used in cellular telephones is increased, smaller SAW apparatuses are used for filters in the communication units. Additionally, multi-functional components are being developed to reduce the size and the number of the individual components in communication units.
In view of the above background, research is being actively conducted on SAW filters provided with a balanced-to-unbalanced conversion function, i.e., a so-called xe2x80x9cbalun functionxe2x80x9d, used in the RF stage of cellular telephones. Such SAW filters are used mostly in Global System for Mobile communications (GSM) cellular telephones.
A known SAW filter having a balanced-to-unbalanced conversion function and an input impedance and output impedance that are substantially the same is shown in FIG. 67.
In the SAW filter apparatus shown in FIG. 67, an interdigital transducer (hereinafter referred to as an xe2x80x9cIDTxe2x80x9d) 101 is provided on a piezoelectric substrate 100. IDTs 102 and 103 are provided on the left and right sides of the IDT 101 (in a SAW propagating direction). With this configuration, a three-IDT-type longitudinally-coupled-resonator-type SAW apparatus having a balanced-to-unbalanced conversion function is provided.
In the above-described SAW apparatus, reflectors 104 and 105 are arranged to sandwich the IDTs 102, 101, and 103 therebetween. Terminals 106 and 107 are provided as balanced signal terminals, and a terminal 108 is provided as an unbalanced signal terminal.
Another type of SAW apparatus, which is provided with a balanced-to-unbalanced conversion function and has an input impedance and output impedance that differ by, for example, four times, is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-117123.
The SAW apparatus disclosed in the above-identified publication includes, as shown in FIG. 68, a first SAW filter device 111 and a second SAW filter device 112 provided on a piezoelectric substrate. An output signal of the first SAW filter device 111 is 180xc2x0 out of phase with an output signal of the second SAW filter device 112. The piezoelectric substrate is not shown in FIG. 68. With this configuration, the above-described SAW filter apparatus provides not only a filtering function, but also a balanced-to-unbalanced conversion function.
The first SAW filter device 111 includes cascade-connected three-IDT-type longitudinally-coupled-resonator-type SAW filters 118 and 124 which are symmetrical to each other with respect to the symmetrical line extending along the SAW propagating direction. That is, the first SAW filter device 111 is defined by two stages of filters.
In the longitudinally-coupled-resonator-type SAW filter 118, IDTs 114 and 115 are arranged to sandwich a central IDT 113 on the left and right sides thereof (along the SAW propagating direction), and reflectors 116 and 117 are arranged to sandwich the IDTs 114, 113, and 115. Similarly, in the longitudinally-coupled-resonator-type SAW filter 124, IDTs 120 and 122 are arranged to sandwich a central IDT 119 from the left and right sides, and reflectors 122 and 123 are arranged such that they sandwich the IDTs 120, 119, and 121.
The second SAW filter device 112 includes a cascade-connected longitudinally-coupled-resonator-type SAW filter 128, which is the same type as the longitudinally-coupled-resonator-type SAW filter 124, and a longitudinally-coupled-resonator-type SAW filter 127. The longitudinally-coupled-resonator-type SAW filter 127 is provided with a central IDT 133 whose phase is inverted (i.e., about 180xc2x0) by inverting the direction of the central IDT 113 of the longitudinally-coupled-resonator-type SAW filter 118.
One terminal 129 of the first SAW filter device 111 and one terminal 130 of the second SAW filter device 112 are electrically connected in parallel to each other, and the other terminals 131 and 132 are electrically connected in series to each other. The parallel-connected terminals 129 and 130 define an unbalanced terminal 108, while the series-connected terminals 131 and 132 define balanced terminals 107 and 108.
In the SAW apparatus having a balanced-to-unbalanced conversion function, the transmission characteristics within the pass band between the unbalanced terminal 108 and each of the balanced terminals 106 and 107 must have equal amplitude characteristics and 180xc2x0-out-of-phase characteristics. Such amplitude characteristics and phase characteristics are referred to as xe2x80x9cthe amplitude balance levelxe2x80x9d and xe2x80x9cthe phase balance levelxe2x80x9d, respectively.
The amplitude balance level and the phase balance level are defined as follows. When the above-described SAW apparatus having a balanced-to-unbalanced conversion function is a three-port device, and when the unbalanced input terminal is port 1 and the balanced output terminals are port 2 and port 3, the amplitude balance level |A| and the phase balance level |B| are defined as follows:
A=|20log(S21)|xe2x88x92|(20log(S31)|xe2x80x83xe2x80x83(1)
B=|∠S21xe2x88x92∠S31|xe2x80x83xe2x80x83(2)
where S21 indicates the transfer factor from port 1 to port 2, and S31 indicates the transfer factor from port 1 to port 3. Ideally, in the pass band of a SAW apparatus, the amplitude balance level is 0 dB, and the phase balance level is 180 degrees.
In the above-described SAW apparatus having balanced signal terminals, the balance levels between the balanced signal terminals are reduced. One of the reasons for this is as follows. The distance (indicated by 109 in FIG. 67) between the electrode finger connected to the balanced signal terminal 106 and the signal electrode finger of the IDT 102 is different from the distance (indicated by 110 in FIG. 67) between the electrode finger connected to the balanced signal terminal 107 and the signal electrode finger of the IDT 103 by 0.5 times the wavelength, which is determined by the pitch of the electrode fingers.
Then, the total capacitance of the electrode fingers connected to the balanced signal terminal 106 is different from that of the electrode fingers connected to the balanced signal terminal 107, and the conversion efficiency between an electrical signal and a SAW also is different between the balanced signal terminals 106 and 107. As a result, the balance levels are reduced.
Accordingly, as shown in FIG. 70, the amplitude characteristics of the frequency output from the balanced signal terminal 106 shown in FIG. 67 were measured by grounding the balanced signal terminal 107. As shown in FIG. 71, the amplitude characteristics of the frequency output from the balanced signal terminal 107 were measured by grounding the balanced signal terminal 106 shown in FIG. 67. The difference between the amplitude characteristics output from the balanced signal terminal 106 and the amplitude characteristics output from the balanced signal terminal 107 is shown in FIG. 69. FIG. 69 shows that there is a large difference between the amplitude characteristics, and this difference causes a reduction in the balance levels.
In the SAW apparatus having cascade-connected filter devices shown in FIG. 68, the polarities of adjacent electrode fingers of two adjacent IDTs are not symmetrical between the first SAW filter device 111 and the second SAW filter device 112. This further reduces the balance levels.
More specifically, in the IDT 113, the portions located adjacent to the IDTs 114 and 115 (indicated by 125 in FIG. 68), i.e., the adjacent outermost electrode fingers between the IDTs 113 and 114, and the adjacent outermost electrode fingers between the IDTs 113 and 115 are ground electrode fingers. However, in the IDT 133, the adjacent outermost electrode fingers between the IDTs 133 and 134 (indicated by 126 in FIG. 68) and between the IDTs 133 and 135 (also indicated by 126 in FIG. 68) are a signal electrode finger and a ground electrode finger. If the polarities of the outermost electrode fingers between the adjacent IDTs are different between the left and right sides, the frequency and the amplitude level of the resonance mode shown in FIGS. 72A and 72B are changed by the conversion between an electrical signal and a SAW.
If a SAW apparatus having a balanced-to-unbalanced conversion function including two longitudinally-coupled-resonator-type SAW filter devices having different combinations of the outer electrode fingers of the adjacent IDTs, as in the SAW apparatus shown in FIG. 68, a change in the resonance mode reduces the balance levels between the balanced signal terminals.
A change in the resonance mode is also produced in a SAW filter apparatus defined by a single longitudinally-coupled-resonator-type SAW filter device, such as that shown in FIG. 73, thereby reducing the balance levels between the balanced signal terminals.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a SAW apparatus having a balanced-to-unbalanced conversion function which has outstanding balance levels between balanced signal terminals by offsetting a difference between the balanced signal terminals, and a communication unit including such a novel SAW apparatus.
According to one preferred embodiment of the present invention, a SAW apparatus includes at least one SAW filter having at least two IDTs arranged on a piezoelectric substrate in a SAW propagating direction, and an input signal terminal and an output signal terminal for the SAW filter. At least one of the input signal terminal and the output signal terminal is connected to a balanced signal terminal, and weighting is provided to at least a portion of electrode fingers of the SAW filter.
With this configuration, by applying weighting to at least a portion of the electrode fingers of the SAW filter, balance characteristics (at least one of the amplitude balance, the phase balance, and the transmission characteristics) between balanced signal terminals can be adjusted. Thus, the balance characteristics are greatly improved.
In the aforementioned SAW apparatus, the above-described weighting is preferably applied to at least a portion of the electrode fingers so as to improve at least one of the amplitude balance level and the phase balance level between a pair of the balanced signal terminals.
The weighting may be applied to a few electrode fingers including the outermost electrode finger of at least one of the IDTs located adjacent to the other IDT.
The weighting may be applied to a few electrode fingers in the vicinity of the outermost electrode finger of at least one of the IDTs located adjacent to the other IDT.
The weighting may be applied to the electrode fingers located within one half of a length in the SAW propagating direction from the outermost electrode finger of at least one of the IDTs located adjacent to the other IDT.
The weighting may be applied to the outermost electrode finger of at least one of the IDTs adjacent to the other IDT.
The electrode fingers located in a portion between the adjacent IDTs may be a ground electrode finger and a signal electrode finger, and the weighting may be applied to at least one of the ground electrode finger and the signal electrode finger.
The weighting may be applied to a signal electrode finger of the SAW filter.
The weighting may be applied to at least part of the electrode fingers of the IDT connected to the balanced signal terminal of the SAW filter.
The phase of at least one of the IDT may be inverted with respect to the phase of the other IDT, and the weighting may be applied to at least part of the electrode fingers of the phase-inverted IDT.
The above-described weighting may be withdrawal weighting.
A dummy electrode may preferably be provided for a bus bar which faces a bus bar connected to the withdrawal-weighted electrode finger.
The weighting may be applied to at least two continuous ground electrode fingers including the outermost electrode finger of at least one of the IDT connected to the input signal terminal and the IDT connected to the output signal terminal, the ground electrode fingers being located such that they are adjacent the other IDT.
A ground connecting portion may be provided to connect the electrode fingers of the adjacent IDTs which are connected to a ground via the dummy electrode.
The above-described weighting may be apodization weighting in which the interdigital length of at least a portion of the electrode fingers is differentiated from the interdigital length of the other electrode fingers.
The above-described apodization weighting is preferably applied at the approximate center of the interdigital length.
The apodization weighting may further be applied to the electrode finger adjacent to the apodization-weighted electrode finger, and a bending dummy electrode may be arranged such that it faces each of the two apodization-weighted electrode fingers.
The apodization-weighted electrode finger may be the outermost electrode finger of one of the adjacent IDTs, and a dummy electrode may be provided for the other IDT such that the dummy electrode faces the apodization-weighted electrode finger.
The dummy electrode may be grounded.
The above-described weighting may be duty weighting in which the duty of at least a portion of the electrode fingers is different from the duty of the other electrode fingers.
In the aforementioned SAW apparatus, the SAW filter may include at least three IDTs, and withdrawal-weighting may be applied to at least one of the adjacent IDTs, and the weighting applied to the IDT on one side of the SAW filter may be different from the weighting applied to the IDT on the other side of the SAW filter.
In the aforementioned SAW apparatus, two SAW filters may be provided in which withdrawal-weighting is applied to each of the SAW filters, and the weighting applied to one of the SAW filter is different from the weighting applied to the other SAW filter.
The SAW filter may include at least three adjacent IDTs, in which apodization-weighting is applied to a few electrode fingers other than the outermost electrode finger of at least one of the adjacent IDTs on one side of the SAW filter, and withdrawal-weighting is applied to the outermost electrode finger of at least one of the adjacent IDTs on the other side of the SAW filter. A dummy electrode connected to a bus bar which faces a bus bar connected to the withdrawal-weighted electrode finger is provided in the withdrawal-weighted portion.
The SAW filter may include at least three adjacent IDTs, in which duty-weighting is applied to the outermost electrode finger of at least one of the adjacent IDTs on one side of the SAW filter such that the duty of the outermost electrode finger is different from the duty of the other electrode fingers, and withdrawal-weighting is applied to the outermost electrode finger of at least one of the adjacent IDTs on the other side of the SAW filter. A dummy electrode connected to a bus bar which faces a bus bar connected to the withdrawal-weighted electrode finger is provided in the withdrawal-weighted portion.
In the aforementioned SAW apparatus, two SAW filters may be provided, in which apodization-weighting is applied to a few electrode fingers other than the outermost electrode finger of at least one of the adjacent IDTs of one of the SAW filters, and withdrawal-weighting is applied to the outermost electrode finger of at least one of the adjacent IDTs of the other SAW filter. A dummy electrode connected to a bus bar which faces a bus bar connected to the withdrawal-weighted electrode finger is provided in the withdrawal-weighted portion.
The SAW filter may be constructed such that it has a balanced-signal-input and balanced-signal-output filtering function.
The SAW filter may be constructed such that it has a balanced-signal-input and unbalanced-signal-output filtering function or an unbalanced-signal-input and balanced-signal-output filtering function.
At least one of the IDTs may be divided into two portions in the direction of the interdigital length of the IDT.
A pair of the balanced signal terminals may be connected to comb-like electrodes of one of the IDTs.
At least one of the IDTs may be divided into two portions in the direction in which the SAW propagates.
A grounded electrical neutral point is not necessarily provided between a pair of the balanced signal terminals.
In the aforementioned SAW apparatus, two SAW filters may be provided such that they have a balanced-signal-input and balanced-signal output filtering function.
The two SAW filters may be arranged such that an output signal of one of the SAW filter is about 180xc2x0 out of phase with an output signal of the other SAW filter, and the SAW filters are constructed such that they have a balanced-signal-input and unbalanced-signal-output filtering function or an unbalanced-signal-input and balanced-signal-output filtering function.
A SAW filter may be cascade-connected to the unbalanced signal terminal.
The SAW filter may be a longitudinally-coupled-resonator-type SAW filter.
The above-described longitudinally-coupled-resonator-type SAW filter may include an odd number of IDTs.
The longitudinally-coupled-resonator-type SAW filter may include three IDTs.
The total number of electrode fingers of at least one of the IDTs of the longitudinally-coupled-resonator-type SAW filter may be an even number.
In the aforementioned SAW apparatus, at least three IDTs may be provided, and the total number of the electrode fingers of at least the IDT connected to the balanced signal terminal being an even number.
In the aforementioned SAW apparatus, three IDTs may be provided, and the total number of the electrode fingers of at least the IDT located at the approximate center of the IDTs being an even number.
At least one SAW resonator may be connected in series to or in parallel with the SAW filter.
The SAW filter may include at least two cascade-connected SAW filter portions.
According to another preferred embodiment of the present invention, a SAW apparatus includes an input IDT having a plurality of electrode fingers and an output IDT having a plurality of electrode fingers. The input IDT and the output IDT are arranged on a piezoelectric substrate in a SAW propagating direction so as to define a longitudinally-coupled-resonator-type. Weighting is applied to an inner electrode finger other than the outermost electrode finger of at least one of the input IDT and the output IDT.
One of the input IDT and the output IDT is preferably connected to a balanced side, and the input IDT or the output IDT connected to the balanced side preferably includes the weighted electrode finger. The SAW apparatus is provided with an unbalanced-to-balanced conversion function.
With this configuration, by providing a weighted electrode finger for at least one of the input IDT and the output IDT, balance characteristics (amplitude balance, phase balance, and transmission characteristics) between output signals, in particular, between balanced output signals, can be adjusted. As a result, the balance characteristics are greatly improved.
In the aforementioned SAW apparatus, the weighted electrode finger may be located within one half of a total width of all the electrode fingers of the corresponding IDT from the outermost electrode finger of the IDT.
At least two ground electrode fingers including the outermost electrode finger of at least one of the input IDT and the output IDT are preferably sequentially provided, the outermost electrode finger being located such that it faces the other IDT.
With this arrangement, for example, the balanced output signals can be easily and reliably set about 180xc2x0 out of phase with each other.
The weighted electrode finger may be preferably set such that it controls the area of a no-electric-field portion provided between adjacent ground electrode fingers of at least one of the input IDT and the output IDT.
One of the input IDT and the output IDT may be connected to a balanced side, and the area of the no-electric-field portion of one of the two balanced IDTs may be substantially equal to the area of the non-electric-field portion of the other balanced IDT.
With this arrangement, the conversion balance from SAW energy to electric energy between the IDTs, in particular, between the output IDTs, can be adjusted, thereby greatly improving balance characteristics.
A first grounded balance electrode finger is preferably arranged to extend toward the weighted electrode finger such that the length of the first grounded balance electrode finger is substantially equal to the length of the weighted electrode finger.
With this arrangement, the first grounded balance electrode finger compensates for the no-electrode-finger portion provided by the shorter weighted electrode finger. Thus, a reduction in the balance characteristics caused by the no-electrode-finger portion is prevented.
A second grounded balance electrode finger may be arranged to extend in a direction different form the direction of the weighted electrode finger such that the length of the second grounded balance electrode finger is substantially equal to the length of the weighted electrode finger, and a bending dummy electrode is arranged to face the second grounded balance electrode finger and the weighted electrode finger.
With this arrangement, by providing a dummy electrode, the transmission characteristics are greatly improved while maintaining a high level of balance characteristics.
According to a further preferred embodiment of the present invention, a communication unit including one of the above-described SAW apparatuses is provided. By including the SAW apparatus having outstanding transmission characteristics, the communication unit also has outstanding transmission characteristics.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.